Clang warning fix : deprecated-increment-bool

This branch fixes a clang warning reported after Standard_Boolean was moved from int to bool.

In file included from /oce/drv/Image/Image_DColorImage_0.cxx:
63:
/oce/src/Image/Image_GImage.gxx:904:16: warning: incrementing
 expression of type bool is deprecated [-Wdeprecated-increment-bool]                  YAxisMirror++ ;
                  ~~~~~~~~~~~^
/oce/src/Image/Image_GImage.gxx:908:16: warning: incrementing
 expression of type bool is deprecated [-Wdeprecated-increment-bool]                  XAxisMirror++ ;
                  ~~~~~~~~~~~^
/oce/src/Image/Image_GImage.gxx:927:16: warning: incrementing
 expression of type bool is deprecated [-Wdeprecated-increment-bool]
                  XAxisMirror++ ;                  ~~~~~~~~~~~^3 warnings generated.